43

Columbia University Department of AstronomyÕDepartment of Physics New York, New York 10027 ͓S0002-7537͑93͒05431-9͔ This report covers the period September 1999 through astronomers, primarily amateur, who do stellar photometry August 2000 and comprises an account of astronomical with small telescopes in their backyards. Columbia personnel research carried out in the Department of Astronomy and the included Patterson, Kemp, and Locastro. They typically ob- Department of Physics. serve a star steadily for a few months, trying to amass the densest possible coverage by stressing long observation and distribution of observers in longitude. This provides a time Faculty and Research Associates were James Applegate, series well suited to the study of periodic signals, and immu- Elena Aprile, Norman Baker, William Craig, Arlin Crotts, nized from the ‘‘aliasing’’ problems inherent in data from a Karl-Ludwig Giboni, Eric Gotthelf, Charles Hailey, Jules single site. Long-time observers are in Belgium, Denmark, Halpern, David Helfand, Stephen Kahn, Marc Kamion- Maryland, Arizona, Illinois, New Zealand, South Africa, and kowski, Laura Kay ͑Barnard͒, Karen Leighly, Lloyd Motz Australia. During this period, new nodes were established in ͑Emeritus͒, Reshmi Mukherjee ͑Barnard͒, Robert Novick Finland, Canada, and California. Most programs involve the ͑Emeritus͒, Frederik Paerels, Joseph Patterson, Kevin Pren- study of cataclysmic binaries, justly famous for the many dergast, Andrew Rasmussen, Malvin Ruderman, Daniel periods present in their light curves. Savin, Edward Spiegel, Wilhelmus van der Veen, and Jac- The most interesting result came from intense coverage of queline van Gorkom and David Windt. V803 Cen, a well-known CV with a spectrum dominated by Lam Hui joined the faculty of the Physics Department and helium. It was found that V803 Cen appears to be an essen- Marcella Carollo joined the faculty of the Astronomy De- tially garden-variety dwarf nova, despite its exotica ͑ul- partment. trashort period, helium composition͒. The outbursts occur The Astronomy Department introduced a Universe Se- very often, about every 23 hr, which is why the pattern mester at the Biosphere 2 and recruited three new faculty for wasn’t noticed before ͑because this is close to the traditional the Columbia West campus, Karen Vanlandingham, Philip 24 hr sampling rate͒. They also found ‘‘superhumps’’ in the Yecko and Catherine Garmany. light curve during the star’s bright state, and managed to Graduate students participating in research were Douglas resolve the orbital and the superhump periods. The latter is Bramel, Tzu-Ching Chang, Xinzhong Chen, Jean Cottam, only 0.4% longer than P , which implies a secondary Alessandro Curioni, Akimi Fujita, Mario Jimenez-Garate, orb ͑mass-losing͒ star of only 0.014Ϯ0.009M ᭪ . Since the sec- Stefano Giovanardi, Eilat Glikman, Ming Feng Gu, JaeSub Ϫ9 ondary is now losing mass at ϳ10 M ᭪/yr, this implies that Hong, Miranda Jackson, John Keck, Tomotake Kozu, M. it is evaporating on a timescale of 107 years. Leutenegger, Yuexing Li, Kaya Mori, Nestor Mirabal, Don Large data sets, comprising typically ϳ300 hr over ϳ60 Neill, Ian Mulvany, John Peterson, Jacob Noel-Storr, Masao Sako, Joshua Spodek, Ben Sugerman, Robert Uglesich, nights, have been collected on many other short-period stars, Leven Wadley. to study accretion-disk precession in CVs. Their study and Undergraduates participating in research were Eve Arm- understanding will keep the CBA busy for years to come. strong, Miles Blanton, Maya Cohen ͑Barnard͒, Yosi Gelfand, Uglesich, Mirabal, Sugerman, and Crotts isolated a Jamila Hussain ͑Barnard͒, Eve Locastro ͑Barnard͒, Scott sample of variable stars, mostly RR Lyraes, in the core of the Schnee, Vincent Schoefer, Will Serber, Dana Stern ͑Bar- globular cluster M3, by use of the technique of image sub- nard͒, Gisela Telis, Jennie Watson-Lamprey and Debra Wil- traction. This demonstrates the extraordinary power of this lensky ͑Barnard͒. technique, in that their sample, obtained in moderate seeing Xinzhong Chen, Ming Feng Gu, and Joshua Spodek re- conditions at the ͑ground-based͒ MDM Observatory’s 1.3- ceived Ph.D. degrees. meter telescope, is more complete than a corresponding Appointments during 1999–2000 were held by Adjunct sample obtained over the same area using the Hubble Space Professors Michael Allison from GISS and Michael Shara Telescope. and Mordecai MacLow from the American Museum of Natu- Helfand, in collaboration with E. Moran ͑Berkeley͒, com- ral History, Postdoctoral Research Scientists Ehud Behar, pleted an extensive analysis of the hard X-ray luminosity per Fernando Camillo, Christina Chiappini, Valeri Egorov, O-star from young stellar populations. They show that the Christian Knigge, Uwe Oberlack, Stephen Lawrence, Louis accretion-powered binary population dominates the inte- Tao and Limin Wang. grated X-ray luminosity in the 2-10 keV band, and that the Van Gorkom continued as Chair of the Astronomy De- luminosity per O-star varies by a factor of ϳ10 among the partment, Paerels as Director of the Columbia Astrophysics galaxies of the Local Group. Contrary to previous work, the Laboratory and Kahn as Chair of the Physics Department. X-ray luminosity is not found to be a function of metallicity: the SMC and M31 have the same value. The implications of 1. STARS & STELLAR EVOLUTION these results for the contribution of starbursts to deep During 1998-2000 the activities of the Center for Back- X-ray source counts and the cosmic X-ray background are yard Astrophysics greatly expanded. This is a network of presented. 44 ANNUAL REPORT

2. X-RAY & ␥-RAY SOURCES served these from the ground using the CTIO 4-meter tele- At low Galactic latitude, establishing the nature of the scope, and confirmed them with new and archival data taken majority of the EGRET sources is a problem that continues with the Hubble Space Telescope. to require intensive multiwavelength observational effort. Gotthelf is studying several X-ray sources at the centers The Columbia group made considerable progress this year of supernova remnants in order to understand the evolution by obtaining several probable identifications. Halpern, Hel- of young neutron stars and their relationship to supernovae. fand, Gotthelf, & Leighly discovered a likely neutron-star/ Gotthelf, G. Vasisht ͑JPL/Caltech͒, and T. Dotani ͑ISAS, SNR counterpart of 3EG J2227ϩ6122 using ROSAT, Japan͒ confirmed that 1E 1841–045, the 12-s anomalous ASCA, and the VLA. An unusual feature of this source is a X-ray pulsar ͑AXP͒ which lies at the center of the supernova highly polarized radio shell with a flat radio spectrum. The remnant Kes 73, is spinning down at a remarkably rapid central X-ray counterpart is probably an energetic young pul- pace. The spin-down rate and flux are exceptionally stable; sar, estimated to be at a distance of ϳ3 kpc from its fitted these findings all but eliminate an accretion origin for the column density. Halpern, Mirabal, M. Eracleous ͑Penn State X-ray emission and strongly favor the ‘‘magnetar’’ model, U.͒, and R.H. Becker ͑U.C. Davis͒ reported a probable coun- with an enormous implied magnetic field of 7ϫ1014 G. terpart of the brightest unidentified EGRET source at inter- Along with D. Chakrabarty ͑MIT͒ and V. Kaspi ͑McGill͒, mediate latitude, 3EG J1835ϩ5918. It is a weak, ultrasoft Gotthelf and Vasisht are monitoring the long-term timing X-ray source with no optical counterpart to VϾ25, and prob- stability of the young anomalous X-ray pulsar in the SNR ably a pulsar that is either older or more distant than the Kes 73 using a set of RXTE observations. These observa- prototype, Geminga. It could be a Geminga-like pulsar, or tions span 2 years and a preliminary phase-connected timing even a recycled pulsar with a very high ␥-ray efficiency. solution confirms the remarkable spin-down stability of this Mukherjee, Gotthelf, Halpern, and Tavani identified one of object. This measurement, still in progress, will yield the all the two EGRET sources in the COS–B field 2CG 075ϩ00 important braking index thus distinguishing among various with a blazar behind the Galactic plane ͑3EG J2016ϩ3657͒. NS energy loss models and placing strong constraints on the A variable optical counterpart of this radio blazar was dis- timing noise and glitches of this young pulsar. covered, but a redshift has not yet been determined. Gotthelf discovered a 300 ms X-ray pulsar associated The MDM Observatory continues to pursue optical after- with the young Galactic supernova remnant Kes 75, one of glows of ␥-ray bursts ͑GRBs͒. Several GRBs were success- the few examples of a shell-type remnant with a central com- fully imaged in 1999–2000. Most notably, the optical after- pact radio core. This serendipitous pulsar, PSR J1846–0258, glow of the second most energetic event, GRB 991216, was was found using RXTE data originally obtained for the pur- discovered at MDM by Uglesich, Mirabal, and Kassin ͑Ohio pose of studying a nearby anomalous X-ray pulsar. The new State U.͒. Continued monitoring of its light curve revealed pulsar was subsequently located to the core of Kes 75 using jet-like behavior. The program at MDM will be intensified archival ASCA imaging data. Timing analysis implies a following the recent launch of the HETE–2 satellite, and the characteristic age of only 700 yrs, consistent with the age of increased event rate reported by the interplanetary network Kes 75, suggesting that PSR J1846Ϫ0258 is the youngest of spacecraft. A continuous update of observations of known pulsar. The rapid spin down of this pulsar is likely the GRBs at MDM Observatory is maintained at http:// result of torques from a large magnetic dipole of strength www.astro.columbia.edu/groupresearch.html. Ӎ5ϫ1013 G, just above the so-called quantum critical field. Oberlack has continued research on the interpretation of PSR J1846–0258 resides in this transitional regime where ␥ 26 -ray line emission from radioactive Al in collaboration the magnetic field is hypothesized to separate the regular with the group at MPE Garching, Germany. An upper limit pulsars from the so-called magnetars. ␥2 derived for the nearest Wolf-Rayet star Vel with CGRO/ Most recently, Helfand, Gotthelf, and R.H. Becker have COMPTEL data constrains models of nucleosynthesis in observed Kes 75 with the Chandra X-ray Observatory using very massive stars. The observed line emission from the the ACIS CCD camera. The high-resolution X-ray image of Cygnus region has been studied in detail within the context the supernova remnant is remarkably similar to that seen in of massive-star formation and refined models for the ex- the radio. The new data set will allow the pulsar to be located 26 pected Al lightcurve for several star-formation histories. more precisely and will likely confirm PSR J1846–0258 as a These models can now be tested with other observables, such Crab-like pulsar, albeit one whose period, spin-down rate, ␣ as Lyman- flux and the size of the observed super-bubbles. and spin-down conversion efficiency are each an order-of- magnitude greater than those of the Crab, most likely as a 3. PULSARS, NEUTRON STARS, AND SUPERNOVAE result of its extreme magnetic field. The association of a Lawrence, Crotts, Sugerman and Uglesich, along with P. shell-type remnant in Kes 75 with a coeval pulsar provides Bouchet and S. Heathcote ͑CTIO͒ discovered the first signs strong evidence that neutron stars are born in supernova ex- of the rapid emergency of ‘‘hot spot’’ activity around SN plosions. Using existing and planned monitoring observa- 1987A due to the collision of the ejecta from the supernova tions with RXTE, Gotthelf, Vasisht, Chakrabarty, and Kaspi explosion as it reached the previously existing circumstellar intend to analyze the long long-term timing evolution of the ring in the nebula around the exploding star. This marks the Kes 75 pulsar. These observations span 2 years and will first such time such multiple sites of secondary explosions likely allow a phase connected timing solution if the spin have been observed in meaningful detail around a newly down proves stable. This measurement could yield a value of forming supernova remnant. The Columbia/CTIO group ob- the braking index and distinguish among various NS energy COLUMBIA UNIVERSITY 45 loss models. Conversely, if timing noise and/or glitches are accretion disk as the cause. A comparison study of the ultra- evident, these provide important probes of NS structure. violet emission lines of some of these objects is underway Gotthelf, Vasisht, B. M. Gaensler ͑MIT͒, and K. Torii with HST. New examples of double-peaked Balmer lines ͑Osaka U.͒ continue their study of AX J1845–0258, a 7-s continue to be discovered, primarily in LINERs. An expla- ASCA pulsar which strongly resembles AXPs. A dedicated nation of this association in terms of the ion torus ͑or VLA search at 5 and 8 GHz, centered on the location of the advection-dominated accretion flow͒ was offered a decade pulsar, revealed a previously unknown young (Շ8,000 yr- ago. and continues to be attractive. old͒ supernova remnant, G29.6ϩ0.1. New ASCA data con- Leighly’s investigation into the astrophysics of Narrow- firmed the dramatic reduction in X-ray flux from the pulsar line Seyfert 1 galaxies ͑NLS1s͒ continued this year. The pri- and reveal a faint X-ray point source, AX J184453.3– mary work during this year, in collaboration with Jules Hal- 025642, within the pulsar’s error circle. This X-ray source is pern, has been the analysis and interpretation of HST spectra surrounded by a partial shell of emission coincident with the from two extreme Narrow-line Seyfert 1 galaxies ͑NLS1s͒ radio remnant. A program to monitor this pulsar for further IRAS 13224Ϫ3809 and 1H 0707Ϫ495. The principal obser- activity is in place with RXTE. If the pulsar flux is found to vational result is that the high-ionization lines, including increase, a new detection of pulsation may provide the criti- CIV, Ly␣, NV and SiV are much broader than the low ion- cal period derivative measurement to determine the nature of ization lines including MgII and H␤. While the trend for the 7-s pulsar. high ionization lines to be broader than low ionization lines ͑ ͒ Along with colleagues M. Roberts McGill , Kaspi, M. has been known previously, these spectra show an extreme ͑ ͒ ͑ ͒ Pivovaroff MIT and N. Kawai ISAS , Gotthelf is studying of this phenomenon. Furthermore, the high-ionization lines Chandra observations of the young supernova remnant are strongly blueshifted; they have nearly no emission to the G11.2–0.3. This SNR is proposed as counterpart of the his- red side of the rest wavelength. Again, the tendency for the torical supernova of A.D. 386, one of only eight historical high ionization lines to be blueshifted compared with the low supernovae recorded in the past 2000 years. It contains a fast ionization lines has been known before, but these spectra ͑ ͒ 65 ms spin-powered X-ray pulsar with a characteristic age show the extreme of this phenomenon. The observational of 24,000 years, much older than the SNR. The Chandra results are most simply compatible with the interpretation ACIS imaging observation cleanly separates the X-ray emis- that the emission lines are produced in a disk-wind system: sion from the pulsar, plerion, and shell. Astrometric and the high-ionization lines are produced in a wind coming off spectral analyses of the pulsar and shell will provide critical the disk, probably accelerated by resonance-line driving as is constraints on the distance and age of the system, thereby inferred in CVs and hot stars, and the low ionization lines are allowing the evidence for an association with SN 386 to be produced in the low velocity material at the base of the wind evaluated. or in the disk itself. The disk is optically thick, so emission is Helfand, Halpern, and Gotthelf obtained two target-of- only seen from the wind coming toward us; hence the strong opportunity observations of the Vela pulsar with the Chandra blueshifts imply terminal velocities greater than 10,000 km/s Observatory 3.5 and 35 days after the largest glitch ever ͑Leighly 2000͒. recorded for this object. The images show a spectacular, The results of the BeppoSAX observation of the bright highly structured nebula; the dominant arc brightens signifi- Narrow-line Seyfert 1 galaxy have been analyzed in collabo- cantly between the two observations. They derive the first ration with Andrea Comastri and the other members of the high signal-to-noise X-ray pulse profile for the pulsar and BeppoSAX collaboration on NLS1s ͑Comastri et al. 2000͒. show that neither the pulse shape nor intensity change over The continuum turns out to be complex and includes a power the month following the glitch. A pointing one year after the law plus a soft excess component; furthermore, there is no glitch is now scheduled to search for changes on longer ti- strong evidence for an emission line at 1 keV previously mescales as energy dissipated in the interior at the time of reported from the ASCA data when the continuum complex- the glitch diffuses to the surface. ity is taken into account. Multiwavelength data obtained during the period 1992– 4. ACTIVE GALACTIC NUCLEI 1998 from the luminous NLS1 RX J2217.9-5941 were com- Halpern and Eracleous are continuing their long-term piled and analyzed by Grupe, Thomas & Leighly 2000. It spectroscopic monitoring of very broad, double-peaked was found that this object was bright in the ROSAT All Sky Balmer lines, which are found preferentially in radio-loud Survey, but had faded by a factor of more than 30 later when AGNs. The profiles of these double-peaked lines are highly it was observed twice by the ROSAT HRI and once using variable on time scales of months to years, a behavior which ASCA. The ASCA data were hampered by low signal to can be exploited to evaluate models for their origin, and to noise and contamination by nearby sources; however, indi- study the dynamics of the accretion process in AGNs. Their cations are that the spectrum is steep, and thus, although the recent work demonstrates that variability of the shapes of the object is deficient in X-rays, it does not appear to be ab- emission lines must be due to dynamical motions, and cannot sorbed. be explained by reverberation ͑light echo͒ effects. They also Several new results were presented at the RXTE work- rejected the binary broad-line region hypothesis, and sce- shop held in NASA Goddard Space Flight Center in March. narios involving bloated stars or ‘‘clouds’’ in randomly in- Power spectra from RXTE monitoring observations of the clined Keplerian orbits. Possibly cyclic behavior in several luminous AGN Fairall 9 and 3C 390.3 were compared. objects appears to favor dynamical or wave motions in the These objects have nearly the same hard X-ray luminosity, 46 ANNUAL REPORT so one naively expects the variability properties to be the to quasars and Seyfert galaxies. The goal is to construct a same. Instead the variability power spectrum in 3C 390.3 is sample of over 1000 objects in order to define the logN-logS significantly steeper based on ROSAT monitoring observa- for different source classes and make a definitive determina- tions made in 1995 and RXTE monitoring observations tion of the contributions of each class to the X-ray back- started in 1999. 3C 390.3 is suspected to have an Advection ground. They are also studying both the clustering of X-ray Dominated Accretion Flow ͑ADAF͒, while Fairall 9 may sources in the fields and the apparently large-amplitude cos- have a thin disk, and the difference in variability behavior mic variance apparent in interfield comparisons; with Scharf, may be a direct consequence of the difference in the X-ray they are also searching for correlations between the field gal- emitting region. axy counts and structure in the diffuse background emission. The results of an observation using BeppoSAX of the Helfand, along with Becker and White, have begun a ma- nearby Seyfert 2 galaxy NGC 6300 were presented. The jor effort to construct a sensitive, high-dynamic range radio RXTE observation revealed a huge equivalent width iron image of the Milky Way. Observations of a 13-degree strip line and flat continuum, providing persuasive evidence that of the plane in the VLA D configuration were completed this NGC 6300 is a bright example of a Compton-thick Seyfert 2 summer; C- and B-configuration data will be collected in the galaxy. The BeppoSAX observation, made 2.5 years later, coming year. In collaboration with a group from the Univer- found NGC 6300 in a much different spectral state. The 2-10 sity of Leicester, they will be mapping the same region with keV flux had increased by a factor of two and the X-ray the XMM-Newton Observatory, providing a dramatic new absorption had become Compton-thin. The AGN was view of stellar birth and death in the Galaxy. strongly detected up to 100 keV with the PDS, and rapid hard X-ray variability was also observed. This result may 6. GALAXIES AND LARGE SCALE STRUCTURE provide evidence that the intrinsic source flux increased dra- Carollo, M. Stiavelli ͑STScI͒, P.T., de Zeeuw ͑Leiden͒, matically between the two observations; however, a decrease M. Seigar ͑STScI͒ and H. Dejonghe ͑Gent͒ performed an in the column density cannot be ruled out. A soft X-ray HST/NICMOS NIR survey in H and J for a sample of 69 spectral component, not detected in the Einstein IPC obser- spiral galaxies for which they already had WFPC2 V images. vation, appeared as well. Results from an optical spectropo- The V-H color distribution of bulges with an R1/4 light pro- larimetric observation were made at the CTIO 4-meter tele- file was found to peak around 1.3 magnitudes, with a sigma scope in June 1999 using a visitor spectropolarimetry of about 0.1 magnitudes. For a solar metallicity, these values module. The continuum is about 0.8% polarized, but there is correspond to stellar ages of about 6Ϯ3 Gyrs. In contrast, no evidence for enhancement near emission lines that would the V-H color distribution of bulges with an exponential light indicate the presence of a hidden broad-line region. profile peaked at about 0.9 and has a sigma of about 0.4 mags. This likely implies significantly smaller ages and/or 5. SURVEYS lower metallicities for a significant fraction of the stars in the Helfand and his principal collaborators R. Becker and exponential bulges compared to the R1/4-law spheroids. R.L. White ͑STScI͒ completed another observing session for Photometrically-distinct nuclei ͑previously detected in the 2 the FIRST survey with the VLA. To date, 7998 deg have optical͒ were detected in the NIR in the centers of many been mapped to a sensitivity threshold of 1 mJy at 20cm and spiral galaxies, including systems with an exponential bulge. 722,000 sources have been located with positional accuracies Most of the central nuclei hosted by the exponential bulges of better than 1ЈЈ. were found to have V-H and J-H colors which are compat- Helfand and Glikman with their FIRST collaborators have ible with relatively unobscured stellar populations. Assuming matched the FIRST radio catalog to the 2MASS catalog of no or little dust effects, ages of about or in excess of 1 Gyr near-IR sources in a search for the putative population of were derived for these nuclei, which in turn imply masses of 6 7 highly reddened quasars. They have found 64 objects de- about a few 10 to a few 10 M ᭪ . This HST photometric tected at both radio and IR wavelengths which are absent on survey has shown that the structures which are currently the POSS-I plates; all have BϪKϾ5. A dozen objects have formed inside the disks are quite dissimilar from the old been imaged at K-band with Keck, and the majority are stel- elliptical-like spheroids which are hosted by the early-type lar. A spectrum of the most extreme object with BϪKϾ8 disks. shows a highly reddened zϭ2.2 quasar that is gravitationally Carollo and S.J. Lilly ͑Herzberg Institute͒ measured the lensed. Followup observations of the remaining objects in emission line ratios in a sample of 34 CFRS star-forming the sample are planned. galaxies with redshifts between 0.5ϽzϽ1.0, and computed Helfand and Kinkhabwala, in collaboration with F. Harri- their metallicities by means of the empirically-calibrated R23 son ͑Caltech͒, have begun a major program to identify ser- metallicity estimator introduced by Pagel & Edmunds endipitous faint X-ray sources in Chandra fields. To date, ͑1979͒. A preliminary analysis, focussed on a high-H␤ se- they have produced a catalog of over 300 sources detected in lected sample of 15 galaxies, showed that the metallicities of the 2-9 keV band in a dozen fields and have obtained deep these galaxies appear to be remarkably similar to those of (Rϳ24Ϫ25) optical images of all these fields. Spectroscopy local galaxies selected in the same way, suggesting that there of the optical counterparts has been completed for about one- has been little change in the relationship between metallicity quarter of the objects and substantial additional observing and line- and continuum-luminosity from redshift about 1 to time has been scheduled. They find a wide variety of coun- today. This work does not support the idea that the 0.5Ͻz terpart types ranging from normal and starbursting galaxies Ͻ1.0 ͑high-H␤) galaxies are dwarf galaxies brightened by COLUMBIA UNIVERSITY 47 large bursts of star-formation, as had been suggested from ͑Chiappini & Matteucci, 2000a; Chiappini et al. 2000͒ and previous studies. Rather, these findings are more consistent concluded that an IMF which is a strong function of time with a picture in which these systems are the progenitors of will not lead to a good agreement with the observational today’s massive metal-rich galaxies. constraints, suggesting that if the IMF varied this variation W. Evans ͑Oxford͒, Carollo and P.T. de Zeeuw ͑Leiden͒ should have been small. The two-infall model was also ap- used scale-free triaxial halo models with flattish rotation plied in the study of the evolution of the light elements in the curves to compute the differential detection rate for energy Galaxy ͑Chiappini & Matteucci 2000b͒; to constrain the deposited by rare WIMP-nucleus interactions. Although tri- mass surface density in the local disk ͑Romano et al. 2000͒ axiality and velocity anisotropy change the total rate by only and to investigate the evolution of oxygen abundance in the about 20%, they have a substantial effect on the shape of Galaxy ͑Matteucci & Chiappini 2000͒. recoil spectrum. In particular, the study showed that the am- Crotts is principle investigator of an international collabo- plitude of the annual modulation signal depends sensitively ration ͑MEGA: Microlensing Exploration of the Galaxy and on the velocity anisotropy, being weaker in the radially an- Andromeda͒ to determine if large, dark matter objects, i.e., isotropic halo models, and that even the sign of the signal MACHOs compose a large fraction of the mass of the An- can be changed. Restricting attention to low energy events, dromeda Galaxy, M31. They recently wrote several papers as some experimental groups do, this study showed that the describing their survey and its utility in solving the spiral maximum WIMP rate could occur in December rather than galaxy halo dark matter problem, and expect to have prelimi- in June. nary results in the near future. G. Verdoes ͑Leiden/STScI͒, R. van der Marel ͑STScI͒, Crotts, along with D. Vanden Berk, C. Stoughton Carollo and P.T. de Zeeuw ͑Leiden͒ studied the kinematics ͑FNAL͒, D. Tytler and D. Kirkman ͑UCSD͒ have shown that of the central gas disk in the counter-rotation-core galaxy many of the redshift clumps of galaxies seen in the Hubble IC1459, and found in this system a black hole mass in the Deep Field are contained in what appear to be large sheets 8 range 1-4 10 M ᭪ . This implies a black hole mass to galaxy ͑more than about 10 Mpc across͒ of gas containing heavy mass ratio in the range 0.4Ϫ1.5ϫ10Ϫ3, which is not incon- elements, e.g., magnesium. In the process of this investiga- sistent with results obtained for kinematically-normal galax- tion, they have provided the most complete known sample of ies. quasars in the vicinity of the HDF. Chiappini and Carollo are studying optical spectra for a Hailey, Craig and Neill have been studying the ‘beta sample of about 50 bulges to derive metallicity/age sensitive problem’ in Abell 262 and have just had a paper accepted absorption indices and emission line fluxes ͑and ratios͒. This ͑Neill et al. 2000͒ with the same title. They used the Auto- study will provide a map of the local stellar population ͑age mated Multiobject Spectrograph at the 3 meter telescope at and metallicity͒ properties in bulges as a function of Hubble Lick Observatory ͑built by Hailey, Craig and J. Brodie at type, and of their formation timescales. The stellar popula- UCSC͒ to obtain a large sample of galaxy redshifts within tion properties will be also investigated as a function of the Abell 262. Combining this data with ROSAT HRI and ISM properties. The sample was selected from the one inves- ASCA data they were able to construct a comprehensive pic- tigated by Carollo et al. ͑1997, 1998͒ with HST. The avail- ture of the cluster. They have shown that there is no beta ability of information on the nuclear scales will allow to problem in Abell 262 ͑as had been previously reported͒ but relate the central properties of bulges ͑nuclei͒ to their large that the actual beta is a factor of two different than that scale properties. previously reported. This completely changes the picture of Chiappini has also been working in a chemical evolution the cluster in terms of the relative temperature of galaxies model for the Milky Way formation. Chiappini, F. Matteucci and gas and has important implications for models of galaxy- ͑U. di Trieste͒ and D. Romano ͑SISSA/ISAS͒͑2000a,b͒ gas interactions. present theoretical results on the galactic abundance gradi- Helfand and Scharf, in collaboration with Lacy ͑IGPP͒, ents of several chemical species for the Milky Way disk, have begun a program to study large-scale structure in the obtained using an improved version of the two-infall model redshift interval 0.2-1.2 using hosts of classical radio galax- of Chiappini, Matteucci and Gratton ͑1997͒ that incorporates ies as tracers of dense regions of the Universe. They are a more realistic model of the galactic disk and halo. This mapping 80 deg2 in I-band at MDM, and will identify ellip- improved model provides a satisfactory fit to the elemental tical hosts of FIRST radio sources for followup spectroscopy. abundance gradients as inferred from the observations and A pilot study showed that with only ϳ30 redshifts, a signifi- also to other radial features of our galaxy ͑i.e., gas, star for- cant spatial clustering was detected; the goal is to obtain mation rate and star density profiles͒. They conclude that a several hundred identifications in order to observe directly relatively short halo formation timescale (Ӎ 0.8 Gyr͒,in the evolution of structure over the redshift interval studied. agreement with recent age estimates for the age differences Helfand, in collaboration with Leighly, E. Blanton ͑UVa͒, among Galactic globular clusters, coupled with an ‘‘inside- and the FIRST team, completed an imaging and spectro- out’’ formation of the Galactic disk where the innermost re- scopic study of 40 bent-double radio sources which form a gions are assumed to have formed much faster than the out- complete, magnitude-limited sample. They find many such ermost ones, represents, at the moment, the most likely objects reside in clusters or groups, with the richest, lower- explanation for the formation of the Milky Way. Chiappini redshift portion of the sample being detected in the ROSAT also explored the effects of adopting an initial mass function All Sky Survey. However, some bent doubles appear in poor variable in time on the chemical evolution of the Galaxy environments, suggesting either that they lie in fossil groups 48 ANNUAL REPORT

͑in which the host has cannibalized all its companions but ies have a wide range of HI properties. Limits on the radio hot intracluster gas remains͒ or that alternative mechanisms continuum emission from these galaxies rule out that these for bending the radio lobes are required. are dust enshrouded starbursts. ͑Chang et al. 2000͒ Helfand and Chang in collaboration with Refregier ͑Cam- M. Balcells ͑IAC, Tenerife͒, van Gorkom and R. Sancisi bridge͒ continued their attempts to extract a weak-lensing ͑Bologna͒ analyzed the HI distribution and kinematics in the signal from the FIRST data. A detection consistent with the shell elliptical NGC 3656. For the first time stellar kinemat- expected signal on scales of 20 – 200Ј is seen, although the ics was obtained for part of a shell. The stellar velocities systematic effects have yet to be fully characterized. They agree with the HI kinematics. Thus for NGC 3656 the HI is are now pursuing a technique which would extract the signal truly associated with the shell. This fits in with a very exten- directly from the uv data, greatly simplifying the correction sive database obtained by D. Schiminovich ͑Caltech͒, van of systematic effects. Gorkom and J. van der Hulst ͑Kapteyn Institute͒ on shell The structure of clusters of galaxies and the evolution of galaxies. Many of these are found to be HI rich and the galaxies in those environments, from the low density outer morphology suggests a common origin of the HI and stellar parts to the dense inner cores is probed in HI by van tidal features. Gorkom. Survey time has been allocated at the VLA to study Giovanardi, van Gorkom with J. Hibbard ͑NRAO͒ and J. a sample of nearby clusters. The project, done in collabora- Stocke ͑Colorado͒ continued their search for HI from the tion with H. Bravo-Alfaro ͑Guanajuato͒, K. Dwarakanath environment of nearby Ly alpha absorbers. Results were ob- ͑RRI, Bangalore͒, P. Guhathakurta ͑UCSC͒, B. Poggianti tained for another 6 absorbers. Only one HI emitter was ͑Padova͒, D. Schiminovich ͑Caltech͒, M. Valluri ͑Chicago͒, found, at large projected distance from the absorber. The M. Verheijen ͑NRAO͒, E. Wilcots ͑Wisconsin͒ and A. Zab- long term goal is to see whether the presence of a Ly alpha ludoff ͑Arizona͒, aims at getting complete HI imaging data absorber affects the HI detection statistics in its environment for 12 clusters covering a redshift range from 0 to 0.2. This and whether anything can be said about possible association will be combined with deep multi color imaging and spec- between absorber and emitters. ͑ ͒ troscopy. A byproduct of the HI observations will be a deep M. Blanton, van Gorkom and S. Baum STScI searched radio continuum image of each of the clusters. This database for HI absorption in a sample of radio loud ellipticals. Sev- will allow for a detailed study of the star formation history eral new detections were made, but the results are more com- ͑and future͒ of galaxies of different morphological types, of plicated than the simple infall picture previously found by ͑ ͒ the interaction and merger rate of the galaxies and of the van Gorkom et al. 1989 . dynamical state of the clusters. Examples of what can be done with this kind of data are the results on the Coma Clus- 7. X-RAY SPECTROSCOPY ter ͑Bravo-Alfaro et al. 2000͒, where clear evidence for sub- Last year saw the advent of high resolution spectroscopy structure is found from the HI data and on A2670 ͑Poggianti in X-ray astronomy, with the successful launches of the and van Gorkom, 2000͒. The study of A2670 is prototypical Chandra and XMM-Newton observatories, both of which for the survey. The data base is identical to what has been carry sensitive, high resolution diffraction grating spectrom- obtained for clusters at intermediate redshift. In A2670 there eters, capable of carrying out observations on a routine basis. is also information about the gas content, information which The Columbia Astrophysics Laboratory, having built the ar- is lacking at intermediate z. Thus the survey will provide an rays of diffraction gratings for the Reflection Grating Spec- ideal comparison sample for studies at intermediate redshift. trometer ͑RGS͒ on XMM-Newton, under the direction of A number of interesting results were obtained for the Steve Kahn, is deeply involved in the calibration and opera- Virgo Cluster. Li and van Gorkom imaged the HI in the giant tions of this instrument, as well as the interpretation of the elliptical M86. This galaxy has an extended asymmetric X first spectra to be obtained with it ͑the instrument team in- ray halo and spatially coincident HI that is kinematically volves Kahn, Rasmussen, Behar, and Paerels, and Cottam, decoupled from the stellar kinematics. They conclude that Sako, Peterson, and Leutenegger͒. the HI has most likely cooled out of the X ray gas ͑Li and It is not an exaggeration to say that this new field of study van Gorkom, 2000͒, after it has been shocked by an ICM- has so far exceeded all expectations. Virtually every spectro- ISM interaction. J. Kenney ͑Yale͒, van Gorkom and B. Voll- scopic observation obtained at Columbia over the last year mer ͑MPA, Bonn͒ have imaged the HI and radio continuum has yielded a surprise, and the results are starting to affect in the highly inclined Virgo spiral, NGC 4522. The images almost every field of observational high energy astrophysics. reveal extraplanar HI, which is sharply cutoff in the disk, and We were able to build on previous experience, gained with a huge non thermal radio plume, stretching out from the disk the analysis of lower resolution CCD spectra, combined with over at least 14 kpc. These distributions look remarkably like a systematic program aimed at the development of the quan- the SPH simulations by Vollmer, which suggests that the titative foundation of astrophysical X-ray spectroscopy. galaxy was stripped on its way through the core of Virgo and Sako, Kahn, Paerels, and their collaborators D. A. Liedahl the current gas morphology shows gas falling back onto the and P. S. Wojdowski ͑Lawrence Livermore National Labo- galaxy. ratory͒ have carried out detailed spectroscopic studies of Chang, van Gorkom, A. Zabludoff and D. Zaritsky ͑Univ massive X-ray binaries using the CCD spectrometers on- of Arizona͒ and C. Mihos ͑CWRU͒ completed a search for board the ASCA Observatory. They have re-examined archi- HI in a sample of EϩA galaxies in the local universe in a val data from Vela X-1 and Cen X-3 in the context of de- variety of environments. The conclusion is that EϩA galax- tailed atomic physics calculations and simple stellar wind COLUMBIA UNIVERSITY 49 models. In Vela X-1, where the X-ray luminosity is much clude, very roughly speaking, that the current understanding lower than in the Cen X-3 system, the stellar wind is clumpy of spectral formation in a collision-dominated plasma ap- and consists of numerous cold clouds embedded in a much pears to be accurate in its dominant features. hotter medium. On the other hand, the stellar wind in Cen Peterson and Paerels, with collaborators at SRON, Saclay X-3 is determined to be more homogeneous. The method is ͑Paris͒, MPE ͑Munich͒, GSFC, and Cambridge University general and applies to a wide class of objects. It has also ͑UK͒, have analyzed the first resolved soft X-ray spectra of been successfully applied to the spectral analysis of the cooling flows in clusters of galaxies, obtained with the RGS, ASCA spectrum of a Seyfert 2 galaxy, the Circinus. In this the only instrument that is currently capable of obtaining system, the authors conclude that the X-ray emission lines wavelength-resolved soft X-ray spectra on these objects. In are formed in a medium extending out to a few hundred the first four strong cooling flow clusters observed, they find parsecs from the center of the galaxy, and derived quantita- that they can spectroscopically trace gas cooling all the way tive constraints on the spatial distribution of material, which down to kTϳ1Ϫ2 keV, but that the gas disappears at lower will be essential for distinguishing between inflow and out- temperatures. The possible explanations for this observation, flow models of AGNs. and the implications for the thermodynamics of cooling gas In the following, we list the highlights from this first year in clusters of galaxies, are currently being investigated. of X-ray astrophysical spectroscopy, based on data obtained Rasmussen, Behar, Cottam, Kahn and collaborators at both with the XMM-Newton RGS, as well as the grating SRON and MPE, started the analysis of a large, systematic spectrometers on Chandra. soft X-ray spectroscopic survey of bright, compact Super- Kahn, Cottam, Sako, and Paerels have studied a sample of nova remnants in the Magellanic Clouds. These objects are accretion powered sources, both galactic and extragalactic, small enough that the RGS can still resolve their soft X-ray with collaborators at the Space Research Organization of the emission line spectra, which opens the possibility to study Netherlands ͑SRON͒, the Mullard Space Sciences Labora- the properties of the hot shocked gas in detail ͑degree of tory ͑MSSL, UK͒, and Lawrence Livermore National Labo- equilibration, excitation mechanism, abundances, velocity ratory. An early spectrum of the massive binary Cygnus X-3 fields, etc.͒. Among the early results from the study is the showed a pure recombination-driven discrete emission spec- finding that in the very young, oxygen-rich SMC remnant trum, with a number of novel spectroscopic diagnostics 1E0102-72.3, the ratio between the Ly␣ emission lines, and clearly resolved for the first time. The edge-on binary the higher corresponding series members, in all the H-like EXO0748-67 likewise showed a novel line spectrum, dis- ions from C VI to Ne X, appear to be well below their values playing emission from high-density photoionized gas which for electron impact excitation, whether in or out of colli- is evidently distributed in a flattened, disk-like geometry sional equilibrium. A possible explanation invokes excitation ͑far͒ above and below the accretion disk. A high resolution by charge exchange on neutral H at the interfaces of cool spectrum of the Seyfert 2 galaxy Mkn 3 shows slightly ex- embedded clouds within the remnant. tended X-ray line emission from the nuclear region, indica- Kahn, Cottam, Leutenegger and collaborators studied the tive of a warm medium in photoionization equilibrium, with high resolution spectrum of the O star ␨ Pup. The RGS spec- parameters possibly in the right range for it to be the ’warm trum is dominated by very strong, Doppler-broadened emis- absorber’ now seen in high resolution spectra of Seyfert 1 sion lines from all abundant elements, up to Si, with particu- galaxies. One such galaxy, IRAS13349ϩ2438, shows evi- larly strong emission from N, indicative of CNO burning. dence for at least two distinct regions of ionization in its The so-called He-like ’triplets’ of N, O, and Ne, indicate that X-ray absorption line spectrum, with kinematics consistent these ions are bathed in the intense photospheric UV field of with an accelerated outflow away from the black hole. the O-star, which measurably affects the level populations Most spectacularly, the Seyfert 1 galaxies MCG-6-30-15 and hence the X-ray emission spectrum. Constraints on the and Mkn 766, in high resolution spectra obtained with the intensity of the stellar UV field, and the kinematics inferred RGS, do not show the expected strong continuum absorption from the line profiles, combine to produce a model for the in partially ionized gas ͑with large absorption edges due to stellar wind and its X-ray emission that is consistent with H- and He-like oxygen, as inferred from CCD spectroscopy͒. current theoretical models to a remarkable degree. Instead, the spectrum appears consistent with relativistically With the RGS, we are now also acquiring the first sensi- distorted line emission from highly ionized oxygen, nitrogen, tive photospheric emission spectra from compact objects. and carbon, arising in material orbiting close to the horizon The first such spectrum was that of the famous source CAL of a massive black hole. 83 in the LMC, a white dwarf which supposedly undergoes Behar, Cottam, Kahn, and collaborators at SRON, MSSL, stable nuclear burning of accreted material at it surface. Paer- and the Paul Scherrer Institute ͑PSI, Switzerland͒ have car- els, Rasmussen, and collaborators at SRON find that the ried out a systematic study of the first few bright coronal spectrum shows very significant spectroscopic structure, spectra, the Chandra High Energy Transmission Grating most likely due to numerous absorption lines from the mid-Z Spectrometer spectrum on Capella, and the RGS spectra of element L-shells. While a quantitative interpretation of this HR1099 and Capella. These spectra, apart from their astro- spectrum must await dedicated radiative transfer calcula- physical interest, constitute a ’benchmark’ of our under- tions, it is abundantly clear from a simple inspection of the standing of X-ray spectroscopy, in arguably the simplest of spectrum that ’blackbody fits’ to spectra of this kind will ͑or astrophysical settings, at least in terms of the relevant exci- should͒ be a thing of the past. Strangely enough, and some- tation mechanism. From these studies these authors con- what disappointingly, the reverse turns out to be true for the 50 ANNUAL REPORT spectrum of the isolated neutron star RXJ0720.4-3125. Paer- can be planned. With its large FOV and good sensitivity in a els, Mori, and collaborators at MPE, MSSL, the University broad energy band, LXeGRIT is well suited for imaging ob- of Strasbourg, and SRON, show that the spectral shape is servations of compact sources in the Galactic Center and close to a blackbody shape, without any strong photospheric bulge, and for a study ͑it would be the first time with an absorption features, which will make it very difficult to mea- imaging telescope͒ of the positron annihilation line and con- sure the fundamental parameters of this object ͑or of any tinuum emission. other isolated neutron star, if this object is at all characteris- Spectral and Compton imaging analysis of the recent tic͒. flight data is in progress and exciting results are expected. Finally, as a byproduct of sensitive spectroscopic studies More information and pictures of the detector and the flight of the Low-Mass X-ray Binary X0614ϩ091 and the Crab campaign can be found at http://www.astro.columbia.edu/ Pulsar with the Low-Energy Transission Grating Spectrom- ϳlxe/lxegrit. Along with a continuation of a flight program eter on Chandra, the first resolved interstellar X-ray absorp- with LXeGRIT, the Columbia team is continuing the re- tion spectra were obtained. Paerels, Kahn, Savin, and col- search and development program on a ‘‘warm’’ liquid xenon laborators at SRON and MPE found narrow oxygen TPC ͑or high pressure gaseous TPC͒, as proposed in their absorption lines in the spectrum of X0614ϩ091, which ap- last NASA High Energy Astrophysics SR&T research pro- pear to indicate the presence of oxygen in at least two gram, to match the requirements of a next generation Ad- phases, atomic and molecular, along this line of sight. Like- vanced Compton Telescope for a sensitive nuclear line astro- wise, Noel-Storr, Paerels, and Kahn identify multiple inter- physics mission. stellar oxygen absorption features in the spectrum of the Crotts has finished construction of a 8192ϫ8192 pixel Crab Pulsar, which are currently under investigation. CCD imager for use on the MDM Observatory’s 1.3-meter and 2.4-meter telescopes. This is one of the largest CCD 8. LABORATORY ASTROPHYSICS & imagers in the world and has already gone into regular ser- INSTRUMENTATION vice on both telescopes. It offers a field-of-view of 45 and 25 Aprile, Curioni, Egorov, Giboni, Hussain, Oberlack, arcminutes, respectively, on the two telescopes, and is well- Zhang and S. Ventura ͑INFN–Padova University, Italy͒ con- matched to the subarcsecond seeing performance, particu- tinued their work on the Liquid Xenon Gamma-Ray Imaging larly on the 2.4-meter telescope. ͑ Telescope ͑LXeGRIT͒ project. A considerable effort has Mukherjee and D. Bramel worked on STACEE Solar ͒ been devoted to data analysis, both of the ’99 LXeGRIT Tower Air Cherenkov Effect Experiment , a ground-based ␥ balloon flight and of laboratory calibration data. The results, detector that is sensitive to high energy -rays in the regime concerning the spectroscoscopic and imaging performance 50 to 300 GeV. This part of the electromagnetic spectrum and the performance at balloon altitude during the ’99 flight, has been largely unexplored and astrophysics in this energy have been presented at the SPIE2000 conference and at the range promises exciting scientific returns. STACEE reported Frontier Detectors for Frontier Physics conference ͑Isola the detection of the Crab nebula, thus demonstrating that d’Elba, Italy͒. lower energy thresholds can be achieved by using existing This extensive analysis work, followed by various im- large arrays of solar heliostat mirrors to collect Cherenkov provements of the instrument, eventually led to a new bal- light. loon flight on October 4, 2000, from Fort Sumner, NM, the Mukherjee, Gotthelf, Halpern, Dana Stern, and M. Tavani main changes being a trigger fully optimized for the MeV ͑IFCTR-CNR͒ worked on unidentified EGRET sources, spe- region, a ‘‘naked’’ LXe TPC ͑the active shield used during cifically the two sources in the COS-B ␥-ray field of 2CG the ’99 flight having been removed͒, fastened data acquisi- 075ϩ00. Work on several of these mysterious unidentified tion and improved on–board data storage. high energy sources is currently in progress. Mukherjee also It has been a great turn–around flight with more than 24 worked on the spectral analysis of several active galaxies hours at float altitude ͑about 27 hours including ascent͒, and observed by EGRET. the LXeGRIT instrument performed perfectly in all its parts. Behar has continued his works on fundamental atomic The flight was terminated on October 5 late afternoon, to physics, investigating quantum interference effects in photo- avoid the risk of a cut-down at night. The main science focus recombination processes, electron density effects on dielec- was imaging and polarization studies of the Crab nebula, but tronic recombination, and spectroscopic analysis of laser other strong sources such as 3C273 or Cygnus X-1 have also produced plasmas of heavy metals. been in the field-of-view. There are very interesting, very Savin, Kahn, and Gu and their collaborators P. Beiersdor- high energy, cosmic ray events in the sample. A total of fer, B. R. Beck, G. V. Brown, D. A. Liedahl, and J. H. almost 40 GB of event data were collected, of which about Scofield ͑Lawrence Livermore National Laboratory͒ have 80% have been stored on–board, and the rest downlinked developed a a technique for producing a Maxwell-Boltzmann through two 500 kbps telemetry channels. The total LXeTPC electron energy distribution using an electron beam ion trap trigger rate, remained nicely constant around 700 Hz, after ͑EBIT͒. The technique was implemented on the Lawrence- the expected increase during ascent. It looks like a great Livermore EBIT to simulate Maxwellian plasmas. To verify success for a new material like LXe in a high radiation en- the fidelity of the quasi-Maxwellian, they have measured line vironment as the nearby space. emission due to dielectronic recombination ͑DR͒ and elec- The payload is now back at the Columbia Astrophysics tron impact excitation ͑EIE͒ of heliumlike neon, magnesium, Laboratory, in perfect working conditions and next flights and argon for a range of simulated electron temperatures. COLUMBIA UNIVERSITY 51

The ratio of DR to EIE lines in heliumlike ions is a well than ͑Si/C͒᭪ or greater than 3͑Si/C͒᭪ . If the latter is true, understood electron temperature diagnostic. The spectro- then it suggests the metagalactic radiation field is not due scopically inferred quasi-Maxwellian temperatures are in ex- purely to quasars, but includes a significant stellar compo- cellent agreement with the simulated temperatures. nent. Lastly, column density ratios of Si IV to C IV versus C At a recent conference, Savin presented a review of the II to C IV are often used to constrain the decrement in the status of dielectronic recombination ͑DR͒ rate coefficients metagalactic radiation field at the He II absorption edge. used for modeling cosmic plasmas. He gave a brief overview Savin shows that the variation in the predicted Si IV to C IV of theoretical and experimental studies of DR. He also ratio due to a factor of 2 uncertainty in the DR rates is almost showed results which demonstrated the astrophysical impor- as large as that due to a factor of 10 change in the decrement. tance of accurate DR rates for studies of the intergalactic Laboratory measurements of the relevant DR resonance medium. strengths and energies are the only unambiguous method to Kahn, Savin, and Gu and their collaborators P. Beiersdor- remove the effects of these atomic physics uncertainties from fer, G. V. Brown, D. A. Liedahl, C. W. Mauche, and S. B. models of the IGM. Utter ͑Lawrence Livermore National Laboratory͒ J. K. Lep- Gu, Savin and P. Beiersdorfer ͑Lawrence Livermore Na- son ͑University of California at Berkeley͒, and J. J. Drake tional Laboratory͒ have developed a theoretical formalism and B. J. Wargelin ͑Harvard-Smithsonian Center for Astro- for calculating the anisotropy and polarization of photon physics͒ reported on recent measurements carried out using emission due to a spiraling beam of electrons colliding with the Lawrence-Livermore Electron Beam Ion Trap. Using an ensemble of atoms or ions. For an axisymmetric beam spectroscopic equipment optimized for laboratory astrophys- with a given velocity angular distribution, the polarization ics, they are performing systematic measurements of the line and angular distribution of the resulting radiation can be emission from astrophysically relevant ions in the wave- characterized by the expansion coefficients of the distribu- length band between 1 and 400 Å important to X-ray mis- tion function in terms of Legendre Polynomials. They sions such as Chandra, XMM-Newton, and EUVE. Obtained present simple expressions for dipole and quadrapole radia- in a controlled laboratory setting at electron densities similar tion and apply the results to the case of an electron beam ion to those found in stellar coronae, the data are used to test trap. spectral modeling codes for accuracy and completeness. The Hailey, Craig, Windt, Koglin, and Jimenez-Garate are effort of Kahn and collaborators includes the compilation of working on the High Energy Focussing Telescope ͑HEFT͒ the iron L-shell emission from 6-18 Å and the iron M-shell balloon telescope. HEFT is ␥-ray ray telescope designed for emission lines from 50-200 Å . Many lines have been iden- operation in the 20-70 KeV energy band and will have sub- tified for the first time, and the measured fluxes from lines arcminute angular resolution. It uses true focussing optics. mission in the spectral modeling codes are assessed. The Columbia is constructing the mirrors for HEFT using measurements also assess the accuracy of line excitation cal- multilayer coated, thermally-slumped glass optics developed culations, including direct electron-impact excitation, dielec- by Columbia and the Danish Space Research Institute. The tronic recombination, and resonance excitation. These mea- detectors for HEFT are CZT pixellated detectors being de- surements yield a calibration of specific diagnostic line veloped at CalTech. Lawrence Livermore National Labora- ratios. tory is working on the aspect reconstruction system. The At a recent conference Kahn and his co-workers P. HEFT team has already reported on the first images ever Beiersdorfer, G. V. Brown, D. A. Liedahl, C. W. Mauche, taken in the hard X-ray band ͑at 28 and 68 KeV͒ using a and S. B. Utter ͑Lawrence Livermore National Laboratory͒ prototype optic ͑Craig et al. 2000͒. Currently Columbia is in and J. K. Lepson ͑University of California at Berkeley͒ pre- the process of building the HEFT flight optics and has al- sented a report on detailed laboratory astrophysics measure- ready constructed the gondola. ments to produce spectral tables for the Fe VII - Fe XXIV Hailey, Craig, Windt, Koglin, and Jimenez-Garate, in col- line emission in the EUV wavelength band. Measurements laboration with DSRI are also working on a project to im- are being carried out using the Lawrence-Livermore electron prove the performance of glass optics for use at higher ener- beam ion trap. gies and with even better angular resolution. Such optics are Savin has carried out a series of model calculations of the particularly suited for use in a Nuclear Line Telescope to photoionized intergalactic medium ͑IGM͒ to determine the observe radioactive Cobalt lines from young type Ia super- effects on the predicted ionic column densities due to uncer- novae. The team has produced the first true image in the tainties in the published dielectronic recombination ͑DR͒ rate gamma-ray band ever taken ͑at ϳ150 KeV͒ which will soon coefficients. Based on his previous experimental work and a be reported in the literature. Columbia is also working in comparison of published theoretical DR rates, he estimates collaboration with CalTech on this project. there is in general a factor of 2 uncertainty in existing DR Hailey, Craig, Hong, and Keck are studying several com- rates used for modeling the IGM. Savin demonstrates that pact galactic sources which emit gamma-rays using the ␥-ray this uncertainty results in factors of ϳ1.9 uncertainty in the Arcminute Telescope Imaging System ͑GRATIS͒ flown by predicted N V and Si IV column densities, ϳ1.6 for O VI, Columbia as well as data from other satellites. Keck’s thesis and ϳ1.7 for C IV. He shows that these systematic uncer- is a study of GRS1758, and using data from GRATIS, tainties translate into a systematic uncertainty of up to a fac- BATSE, SIGMA, ROSAT, RXTE, VLA and ASCA he has tor of ϳ3.1 in the Si/C abundance ratio inferred from obser- constructed a comprehensive picture of the source over a vations. The inferred IGM abundance ratio could thus be less long time baseline. He has been able to constrain model pa- 52 ANNUAL REPORT rameters for various theories of emission, in particular find- 200 keV ͑J. App. Phys., 2000͒, and have been successfully ing strong support for Advection Dominated Accretion Flow deposited onto thermally-formed glass mirror substrates. theories. A paper will be submitted this fall on the results. Other multilayer structures composed of Ni and Ni-alloys are Hong’s thesis is on GRATIS observations of 4U1700 and also being investigated ͑Proc. SPIE, 2000͒. Multilayers com- addresses the various conflicting temperature measurements posed of Mo/Si have been developed for mirrors that operate of the source. A paper will be submitted this fall on this near normal incidence in the soft X-ray band. A complement anlaysis as well. of six multilayer ͑Mo/Si͒ mirrors that operate over the range Hailey, Mori, Craig, and Serber have also been working 17-21 nm have been coated for the TXI solar physics sound- on a completely novel concept for detecting antiprotons and ing rocket experiment being developed by L. Golub at SAO. antiHelium. The technique involves the slowing down and Other material combinations for use at much shorter wave- capture of antiparticles in antiatoms with the subsequent de- lengths are also being investigated. Such coatings could en- tection of characteristic X-rays unique to the antiatoms able the development of diffraction limited X-ray telescopes formed. This technique provides an extremely clean positive that operate at 0.3 - 0.5 keV near normal incidence, for milli- signature for the detection of antimatter, and preliminary cal- arcsecond X-ray imaging of stellar coronae, interacting bina- culations indicate it is much superior to alternative tech- ries, jets, etc. Such mirrors might ultimately be used for niques for detecting low energy antiprotons and analysis is X-ray interferometery with micro-arcsecond resolution. also indicating the technique may be much better than use of magnetic spectrometers for detection of antiHelium. A paper 9. OTHER THEORETICAL INVESTIGATIONS will be submitted on the preliminary analysis this fall. Jimenez-Garate, together with J. Raymond ͑CfA͒,D. Hong, Hailey and Craig have been working on direct de- Liedahl ͑LLNL͒, C. Hailey and B. Goldstein ͑LLNL͒, are tection of dark matter. They are members of the United modelling the X-ray line emission from accretion disks illu- ͑ ͒ Kingdom Dark Matter collaboration UKDMC and are minated by a neutron star in low-mass X-ray binaries building the one ton liquid scintillator veto shield for the ͑LMXB͒. These computer simulations will allow them to ZEPLIN III dark matter detector currently under construction interpret the high-resolution X-ray spectra from XMM- in England. Working with Y. Gelfand they have made mea- Newton and Chandra telescope observations in terms of a surements to optimize the light collection of the shield de- physical picture of the accretion disk, its atmosphere, and sign. The veto shield and ZEPLIN detector will be integrated corona. They improved upon Raymond’s ͑1993͒ disk model next spring and then begin operating in the Boulby mine in and use the Hebrew-University/Lawrence-Livermore Atomic England. Hong, Hailey and Craig, in collaboration with U. Code ͑HULLAC͒ to obtain thousands of recombination Sheffield have also completed studies of scintillation effi- X-ray lines for the entire disk, also using a new disk- ciency of liquid scintillators for dark matter detection. They structure code. These lines should be detectable in high- have shown that liquid scintillators display a hitherto unex- inclination LMXB, when direct emission from the neutron pected increase in scintillation efficiency at low recoil energy star is obscured in a ‘‘dip.’’ They found that low-inclination which make them possible dark matter detectors. They had LMXB do not have easily detectable lines. Evaporating disks previously been rejected because of the belief that their scin- have clear spectral signatures ͑observable in dippers͒, and a tillation efficiencies were too small at low recoil energies. A thermal instability occurs in the gas at a critical depth. The paper has been submitted on this work ͑Hong et al. 2000͒. disk structure, now including an atmosphere near hydrostatic Hong, Craig and Hailey continue their research on the use equilibrium, is more flared than previously thought, and it of neutron shields for space-based gamma-ray telescopes. matches the disk thickness and X-ray albedo calculated using They have completed a series of extensive tests of neutron optical light-curve observations ͑de Jong et. al. 1996͒. Their shields using monochromatic neutrons and obtained excel- model also matched most features from the recently obtained lent agreement between experiments and results of Monte- spectrum of the EXO 0748-69 dipper LMXB ͑Cottam et. al. Carlo analysis. They have shown that such shields could be 2000͒. of great utility in future space missions such as EXIST. Their Mori and Hailey have been developing models for im- most recent results have just been published ͑Hong, Craig, & proved analysis of isolated neutron stars in anticipation of Hailey 2000͒. new Newton and Chandra data. In particular a multi- Craig, Hailey and Hong continue to collaborate with Josh configurational Hartree-Fock code for arbitrary B-field has Grindlay at Harvard on the EXIST All-Sky Sky Gamma-ray been developed which treats higher order Landau levels in Survey mission study ͑Grindlay et al. 2000͒. They partici- perturbation theory. The code is faster than previous codes pated in the recent mission study done at NASA/GSFC. available to generate relevant matrix elements for neutron Hong has also developed an improved scheme for imaging star calculations. In addition there are substantial discrepan- using coded aperture masks which solves some fundamental cies with previous work which have been uncovered. This limitations in currently employed schemes. A paper on this work will be reported in a paper to be submitted this fall as work will be submitted this fall. well as at the HEAD meeting in November. Windt is performing research directed at the development A model for the central engine for cosmic gamma-ray of high-performance multilayer coatings for high-energy as- bursts has been developed by Ruderman with Tao and W. trophysics and solar physics. Depth-graded multilayers com- Kluzniak ͑Copernicus Astronomical Center͒. It is based upon posed of W/Si, for use in grazing-incidence hard X-ray tele- the accretion-induced collapse of magnetized (106G͒ scopes, have been shown to operate at energies in excess of O-Ne-Mg white dwarfs. Such events should result in the COLUMBIA UNIVERSITY 53 birth of 1012 G millisecond-spin-period neutron stars with tion studied is the fate of the gluing bifurcation of the un- strong differential rotation: their inner cylinders spin more forced system when a parameter is slowly varied. The system slowly than their outer ones. Toroidal wind-up of magnetic finds a way around the periodicity constraint by a period- field by these differently spinning cylinders with radially in- doubling scenario. A publication is being prepared. creasing spin-rates may lead to transient repeated explosive Fujita, MacLow, A. Meiksin ͑Edinburgh͒ and A. Ferrara pulsar-like subpulse emission with properties needed for ͑Arcetri͒ have modeled the effects of repeated supernova ex- GRB central engines. Development continues on a model for plosions in high-redshift dwarf galaxies. They use ZEUS-3D, radiopulsar glitches which is not based upon the otherwise a second-order, Eulerian, astrophysical gas dynamics code. needed, but controversial pinning and sudden collective un- The goal is to study the role of cosmological feedback in pinning of quantized vortex lines in the crustal superfluid galaxy formation and in metal enrichment of the IGM. A neutrons. The new model is based upon the interactions be- publication is in preparation on parameterizing the rates of tween magnetic flux tubes and neutron vortex lines in the mass loss, energy output and metal ejection from starbursts stellar core and the pull on the crust of stretched core field using the local models. These parameters will then be incor- which penetrates through it. porated into a cosmological simulation, using a n-body/SPH Chen and Spiegel have reexamined the evaluation of ra- code, GADGET, with GRAPE6. diative bulk viscosity ͑RBV͒, whose interest stems from con- jectures that have been made concerning its possible role in PUBLICATIONS causing an inflationary cosmic expansion. The older methods Aprile, E., Curioni, A., Egorov, V., Giboni, K.-L., Oberlack, use an iteration procedure that limits the results to very short U., Ventura, S., Doke, T., Takizawa, K., Chupp, E.L., & mean free paths. When this aspect of the procedure is Dunphy, P.P. 2000, ‘‘A Liquid Xenon Time Projection avoided, and the influence of electron scattering included, a Chamber for Gamma-Ray Imaging in Astrophysics: larger RBV than has been found previously is obtained, one Present Status and Future Directions,’’ in Proceedings of that seems to produce a significant inflationary phase in the Frontiers Detectors for Frontier Physics, Isola d’Elba, contexts of both relativistic cosmology and a version of Italy, May 2000, Nucl. Instr. and Methods A ͑accepted͒. Newtonian cosmology. Aprile, E., Oberlack, U.G., Curioni, A., Egorov, V., Giboni, Since the approach used in that work involves a modifi- K.-L., Ventura, S., Doke, T., Kikuchi, J., Takizawa, K., cation of the standard asymptotic procedures of kinetic Chupp, E.L., & Dunphy, P.P. 2000, ‘‘Preliminary Results theory, it seemed advisable to seek a check on the method by from the 1999 Balloon Flight of the Xenon Gamma–Ray ͑ applying it to classical kinetic theory. Chen, H. Rao Micro- Imaging Telescope ͑LXe GRIT͒,’’ in Proceedings of SPIE ͒ electronics Lab, Columbia and Spiegel have looked at the X-Ray and Gamma-Ray Instrumentation for Astronomy, derivation of fluid dynamics from the BGK model of kinetic eds. K.A. Flanagan and O.H.W. Siegmund, SPIE, Vol. theory, again finessing the iterative procedure that is the hall- 4140, 35. mark of the Chapman-Enskog method. They obtained fluid Aprile, E., Curioni, A., Egorov, V., Giboni, K.-L., Oberlack, equations that are more general than the standard Navier- U.G., Ventura, S., Doke, T., Kikuchi, J., Takizawa, K., Stokes equations, but reduce to them in the limit of zero Chupp, E.L., & Dunphy, P.P. 2000, ‘‘Spectroscopy and mean free path. The agreement of results of the new equa- Imaging Performance of the Liquid Xenon Gamma-Ray tions with experimental findings on shock structure and the Imaging Telescope ͑LXeGRIT͒,’’ in Proceedings of SPIE propagation of ultrasound is clearly better than for the results X-Ray and Gamma-Ray Instrumentation for Astronomy, obtained from the Chapman-Enskog method and a first ac- eds. K.A. Flanagan and O.H.W. Siegmund, SPIE, Vol. count of this has been published. 4140, 36. Together with A. Birch and A. Kosovichev ͑Stanford͒, Aprile, E., Curioni, A., & Oberlack, U. 2000, ‘‘Prospects for Tao, and Spiegel have studied the instability of magnetoa- Nuclear Astrophysics with a High Sensitivity Xenon coustic modes in optically thin radiating atmospheres. They Compton Telescope’’ in Proc. Astronomy with Radioac- find avoided crossings of modes and are led to suspect that tivities, Sep. 29-Oct 2, 1999, SchlossRingberg, Kreuth, negative energy modes arise in this context. This is aimed to Germany, MPE Report 274 ͑ISSN 0178-0719͒; eds. R. be a case study and it has the advantage of bringing out Diehl, D. Hartmann ͑in press͒. clearly the nature of the effects of crossings. A paper has Audard, M., Behar, E., Gu¨del, M., Raassen, A.J.J., Porquet, been submitted for publication. D., Mewe, R., Foley, C.A., & Bromage, G.E. 2000, ‘‘The Spiegel has continued in his collaboration with S. Talon XMM-Newton View of Stellar Coronae: High-Resolution ͑Montreal͒ and F. Paparella ͑La Jolla͒ on the study of mixing X-Ray Spectroscopy of Capella,’’ A&A ͑submitted͒. by overstable convection such as might occur in semiconvec- Balcells, M., van Gorkom, J.H., & Sancisi, R. 2000, ‘‘Shells tion in stars. The oscillatory convection characteristic of this in the HI-rich Elliptical Galaxy NGC 3656,’’ AJ ͑submit- process induces a horizontal shear, as in steady convection, ted͒. but the combination of this large scale flow with oscillatory Behar, E., Cottam, J., and Kahn, S.M 2001, ‘‘The Chandra convection makes for very effective mixing. An account of Iron-L X-Ray Line Spectrum of Capella,’’ ApJ ͑in press͒. this work is in preparation. Behar, E., Rasmussen, A.P., Griffiths, R.G., Dennerl, K., Au- Spiegel has been working with C. Pasquero ͑Turin͒ and dard, M., & Aschenbach, B. 2000 ‘‘High-Resolution C. Tresser ͑IBM͒ on a study of the behavior of a periodically X-Ray Spectroscopy and Imaging of Supernova Remnant forced nonlinear oscillator ͑the Duffing problem͒. The ques- N132D,’’ A&A ͑submitted͒. 54 ANNUAL REPORT

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